1. Field of Invention
This invention relates to direct drive motor systems for driving the joints of multi-joint robots.
2. Description of the Prior Art
In the prior art, the main type of drive system used for driving the joints of a multi-joint robot comprises a DC motor and a decelerator. In such a system, low speed and large torque are produced.
However, the ideal system for such use is one that uses a direct drive (abbreviated "D.D.") system utilizing a motor of the inductor type because of such properties as the life of the brushes, the decelerator of the D.C. motor, and the necessity for maintenance of lubrication oil.
A circuit for driving such a motor is known wherein an exciting current, passing through the coil of the motor, is detected by means of an electric current detecting circuit. Then, the difference between the detected current and a predetermined instructed current level is supplied to an electric amplifying circuit, wherein an exciting current is passed through the motor coil in such a manner that the differential signal becomes zero by means of a pulse width modulation signal (abbreviated PWM).
In this type of driving circuit, the current detecting circuit preferably has a high degree of accuracy and insulating capability and a simple structure.
A type of means for detecting the rotation of such a motor is known in which an optical rotary encoder or a magnetic resolver is used. The means for detecting the rotation of the motor is preferably capable of detecting, at high resolution, the rotational position, rotational speed and position of the magnetic poles of the motor. Furthermore, the means for detecting the rotation of the motor is preferably capable of detecting easily the original point of the rotational position.
As a circuit for controlling the rotation of the motor, a device is known wherein the rotational speed of the motor and the rotated position are controlled in a feedback manner in response to a detection signal from the rotation detecting means. A control circuit of the type described above is preferably capable of adjusting the servo systems in accordance with the conditions for use of the motor, for example, the characteristic frequency of the motor or load inertia.
As a device for stopping the motor, one type is know wherein the coil of the motor is separated from the driving circuit when the motor is stopped so as to generate a short circuit, whereby the motor is stopped by the resistance of the coil consuming kinetic energy due to Joule's effect. When the motor rotates at a high rotational speed, the phase difference (abbreviated as "phase .phi.") between the exciting current and the exciting voltage becomes large due to the inductance of the coil. Thus, the kinetic energy cannot be consumed efficiently. The motor which is used for driving the joints of the robot is stopped at a variety of rotational speed ranges. Thus, the above described stopper device is not suitable for such use.
It is known to use rectangular slits in an optical encoder as the rotational detecting means However, since such a rectangular slit creates a spatial distribution of the passing light in the form of a rectangular shape, means for detecting the passing light receives the rectangular distributed light. Thus, the detection signal includes in principle higher harmonics. If the detection signal is used for controlling the motor, ripples are included in the position and speed signals. As a result, the motor cannot rotate smoothly
The direct drive motor system needs to satisfy many factors. The prior art does not have any system which can simultaneously satisfy all of the above desired properties.